We always told her that is OK to turn back when we're climbing mountains. In the Grand Canyon this was not an option.
We noticed that the biggest problem with kids is that they get bored easily. What we find interesting and stunning, they may not, so we have to keep her engaged with little games and small challenges to brake the monotony of the hikes.
We also told her that Mom and Dad will go hiking and climbing with or without her. She can stay with the grandparents, we're not forcing any activity that she may not enjoy. She always picked us.

Small rewards are offered when completing a tough hike.

On Langley Mom carried a cupcake to the top, on Whitney she got a strawberry milk and for her birthday she got to go to the American Girl Doll store.

We did some training hikes for the RTR but she was exited and ready to do it, especially when she heard about the Best Lemonade In The World at the bottom of the canyon.
Try to find something that your kids like and offer as a reward.
It's work for them to do those hikes and the may want to see some return. Is it a bribe? I don't work for free, do you?
We just got back from the Sierra where we hiked White Mountain and got to Iceberg Lake to attempt Mt. Russell.
She worked me over and got 20$ out of me. The small rewards are getting bigger.
Trip report coming soon.